One type of electrosurgical procedure is known as “underwater” electrosurgery, in which the instrument is operated submerged in an electrically conductive fluid such as normal saline. Even in dry field electrosurgery, the surgical site is often wet, either due to the presence of blood or irrigating fluid. There is often therefore a requirement for the electrosurgical instrument to be sealed such that fluid does not penetrate the instrument and reach the internal components. Fluid ingress can cause unwanted arcing between components, and for this reason electrosurgical instruments are frequently covered with a fluid impermeable sheath. However, even with such a sheath, unwanted fluid ingress can still occur.